1. Field of the Invention
This invention relates to a method for making upset ends on metal pipe, and more particularly to the method of making an internal upset end on metal pipe and the resulting product.
2. Description of the Prior Art
Metal pipe such as drill pipe is normally assembled by joining adjacent lengths of drill pipe. The ends of the drill pipe are normally upset for welding to a threaded tool joint for connection of adjacent pipe lengths. The drill pipe lengths prior to forming of the upset ends and threads are of a constant well thickness and have uniform inner and outer peripheries.
A conventional method of forming an upset end utilizes a die and mandrel to form in a forging operation an external upset portion of increased wall thickness. Then, in a separate step with another die and mandrel, the end of the pipe is pressed or squeezed inwardly to form an internal upset portion. However, the internal upset portion is not formed against the mandrel or constrained by the mandrel, and it is difficult to control the shape of the transition section between the central portion of the drill pipe and the upset end of increased wall thickness. Further, it is difficult to control the length of the transition section which has a varying wall thickness. Stress concentrations usually occur in the transition section to provide high stress concentration factors (S.C.F.). The transition section heretofore has been formed of a taper and it has been found desirable to have a long length transition section to minimize stress concentrations.
A common fatigue failure today is from rotary bending in a section of the drill hole in which there is a change in direction of hole angle. Under a bending moment, fatigue cracks are initiated at the external surface of the pipe body where a minimum wall thickness is provided and in the transition section of the internal upset as stress concentrations occur in the transition section. In the pipe body, the bending stress at the external surface is always higher than that of the internal surface. However, in the transition section or area, the stress at the internal surface may exceed the stress at the external surface as a result of excessive stress concentrations. Thus, fatigue cracks are indicated at the transition section of the internal upset. For design purposes, the transition section should be designed so that the bending stress at the transition section never exceeds the bending stress at the external surface of the pipe body. This problem was considered in a paper presented in 1988 to the International Association of Drilling Contractor (IADC) and Society of Petroleum Engineers (SPE) designated as paper IADC/SPE 17206, and entitled "Appropriate Design of Drillpipe Internal Upset Geometry Focusing on Fatigue Property" authored by Y. Tsukano, S. Nishi, H. Miyoshi and Y. Sogo.
A finite element analysis was utilized for stress analysis of the drill pipe subjected to bending moment. A purpose of the study including the finite element analysis was to obtain the appropriate internal upset geometry and full size fatigue tests were carried out. The study was conducted on a 5 inch diameter pipe having a weight of 19.5 lbs. per foot. The finite element analysis was evaluated by full size fatigue tests and excellent correlation was achieved. In order to characterize the internal upset geometry, two parameters were adopted, i.e. the internal taper length (Miu) and the radius of curvature (R). FIG. 9 of the paper shows a measurement of the internal upset geometry in which the length of the transition section (Miu) is 97.50 ml (3.838 inches) and the radius (R) at the juncture of the transition section with the body of the pipe has a length of 297.53 ml (11.714 inches). The stress concentration factor decreases with an increase in length of the internal taper as shown particularly in FIG. 5 of the paper. Likewise, an increase in the length of the radius results in a decrease in the stress concentration factor as shown in FIG. 5.
U.S. Pat. No. 4,845,972 dated Jul. 11, 1989 is directed to the method for forming the internal upset and steel drill pipe as presented generally in the above paper IADC/SPE 17206. The '972 patent states it is difficult to control the shape of the internal upset portion which includes the taper as this portion is not restrained by the mandrel. Underfill and buckling are apt to occur at the transition area or taper of the drill pipe. The steel pipe is formed by a method of external upset forging and a method for pressing the external upset portion by means of an internal upset die. Two sets of upset dies and mandrels are utilized in this method. As shown particularly in FIG. 2(C), for example, a radius of curvature is shown at a starting point 5b of the internal upset portion with the taper shown at 5 having a taper length of 5a. The radius shown at 5e forms a juncture of the taper with the body of the pipe. Thus, only a very small portion of the entire transition section or area has a radius of curvature.
U.S. Pat. No. 5,184,495 dated Feb. 9, 1993 illustrates a method of internally and externally upsetting the end of a metal pipe utilizing four separate forging steps and one reheating step after the first two forging steps. A transition section between the upset portion and the inner wall of the pipe has a conical internal surface. Two forging steps are used to initially upset the pipe externally and after reheating of the pipe two subsequent forging steps are used to upset the pipe internally to form the finished shape defining an internal conical surface for the transition section. An internal radius of curvature of a relatively small axial length connects the internal conical surface and the internal wall surface of the pipe.